Oligonucleotides and polynucleotides are of interest for development of new products for diagnostics, imaging and therapy, due to their ability to hybridize specifically to oligonucleotides of complementary sequence. This property of single-stranded oligomers (i.e., to locate the complementary sequence and form a double-stranded molecule or a complex as a third strand) can be used to advantage in radiopharmaceutical development. For example, oligon, leotide DNAs are currently under investigation for antisense applications (Uhlmann E., Peyman A., Chem. Rev., 90: 543-584; 1990; and Dewanjee M. K., Diagn. Oncol., 3: 189-208, 1993.). If radiolabeled, these oligonucleotides may usefully deliver radioactivity to targeted cells or tissues. Recently, c-myc oncogene mRNA was targeted in mice with a radiolabeled antisense probe (Dewanjee M. K., Ghafouripour A. K., Kapakvanjwala M., Dewanjee S, Serafini A N, Lopez D M, Sfakinakis G N. J. Nucl. Med., 35: 1054-1063, 1994). Other possible applications include methods of radiolabeling large molecules by hybridization, pretargeting approaches based on oligonucleotides (Kuijpers W H A, Bos E S, Kaspersen F M, Veeneman G H, van Boeckel CAA., Bioconj. Chem., 4: 94-102; 1993) and the amplification of radioactivity within a tumor or other lesion by sequential administration of complementary DNAs (Hnatowich D. J., Winnard P. Jr., Virzi F. Fogarasi M., Sano T, Smith C L, Cantor C R, Rusckowski M., Proceedings, Fifth Conference on Radioimmunodetection and Radioimmunotherapy of Cancer, Princeton N.J., 1994).
Methods for labeling medically important macromolecules with metal radionuclides have been developed, but they have not yet proved generally useful because of limitations in various aspects of their use. The practitioner chooses a particular protein or nucleic acid on the basis of its properties of affinity for a particular target in a biological system, and this affinity is the basis for that molecule's potential as a therapeutic or diagnostic composition. The labeling method must preserve the native structure of that molecule to assure that the binding function is not substantially reduced. Further, the potential use is enhanced if the radionuclide as supplied to the macromolecule is not substantially further transferred to miscellaneous cells or serum proteins, causing diminishment of radioactivity delivered to a target site, and increased non-specific background radiation. The ability to retain the radiolabel is a function of several factors, including the stability of the macromolecule to endogenous enzymes, possible chemical compromise to its structural integrity following the labeling procedure, and the nature of the chemical bond between the macromolecule and the radiolabel.
Proteins have been labeled with technetium-99m (.sup.99m Tc) using the hydrazino nicotinamide (SHNH) chelator (Abrams M. J., Juweid M., tenKate C. I., Schwartz D. A., Hauser M. M., Gaul F. E., Fuccello A. J., Rubin R. H., Strauss H. W., Fischman A. J, J. Nucl. Med., 31: 2022-2028, 1990) and the label found to be stable in vitro and in vivo (Hnatowich D. J., Mardirossian G., Ruscowski M., Fogarasi M, Virzi F, Winnard P Jr., J. Nucl. Med., 34; 109-119, 1993). The SHNH chelator was initially used for oligonucleotides, however, transfer of label nonspecifically to proteins from oligonucleotides labeled in this manner was observed. The identical oligonucleotide; radiolabeled with .sup.111 In using the chelator diethylenetriamine-pentaacetic acid (DTPA) showed no tendency to bind to serum proteins under circumstances in which the .sup.99m Tc-SHNH-labeled oligonucleotide was largely protein bound (Hnatowich D. J., Winnard P. Jr., Virzi F, Fogarasi M, Sano T, Smith C L, Cantor C R, Rusckowski M., J. Nucl. Med., 36: 2306-2314, 1995). This nonspecific protein binding can be attributed to use of the SHNH chelator for coupling the nucleic acid to .sup.99m Tc.
The N-[N-[N-[(benzoylthio)acetyl]glycyl]glycyl]glycine (MAG.sub.3) chelator of .sup.99m Tc was originally developed as an alternative to radiolabeled hippuran for renal function studies (Fritzberg A R., Kasina S., Eshima D., Johnson D. L., J. Nucl. Med., 27: 111-116; 1986). This succinimide ester mercapto-acetyl tripeptide is protected against disulfide-bond formation by a benzoyl group, which must be heated to 100.degree. C. for 10 min during labeling to remove the protecting group. This benzoyl-protected chelator has also been used to radiolabel antibodies with .sup.99m Tc (Fritzberg A. R., Berninger R. W., Hadley S. W. et al., Pharmaceutical Res., 5: 325-334; 1988) and radiorhenium (Goldrosen M H., Biddle W C., Pancook S. Bakshi S., Vanderheyden J-L., Fritzberg A. R., Morgan A. C., Foon K. A., Cancer Res., 50: 7973-7978; 1990). However, past use of the chelator for protein labeling has been limited since the benzoyl protecting group requires extreme alkaline pH or boiling temperatures for sulfur deprotection. The MAG.sub.3 chelator has also been used to label antibodies by post-conjugation methods through the use of an isophthaloyl group for protection in place of the benzoyl group (Weber R. W., Boutin R. H., Nedelman M. A., Lister-James J., Dean R. D., Bioconjug. Chem 1:431-437, 1990). However, in addition to a complicated synthesis, this approach requires an additional step (deprotection) and the immediate labeling of the deprotected-conjugated antibody before disulfide bond formation can occur in solution. Accordingly, this chelator has been radiolabeled prior to conjugation (i.e. preconjugation labeling) with macromolecular polymers such as proteins or polypeptides, which cannot withstand harsh conditions. Preconjugation labeling can be a complex procedure with multiple intermediate purification steps. More importantly from the point of view of application of radionuclide-labeled macromolecules for diagnostics, imaging and therapeutics, preconjugation labeling limits the usefulness of the product: the preconjugation radionuclide-chelator complex has a short half-life, cannot be transported without necessary precautions for radioactivity, and exposes end-users to radioactivity during a number of complex synthetic steps, all required prior to end use with samples or patients.